Molecular characterization of Ph1 as a major chromosome pairing locus in polyploid wheat.

The foundation of western civilization owes much to the high fertility of bread wheat, which results from the stability of its polyploid genome. Despite possessing multiple sets of related chromosomes, hexaploid (bread) and tetraploid (pasta) wheat both behave as diploids at meiosis. Correct pairing of homologous chromosomes is controlled by the Ph1 locus. In wheat hybrids, Ph1 prevents pairing between related chromosomes. Lack of Ph1 activity in diploid relatives of wheat suggests that Ph1 arose on polyploidization. Absence of phenotypic variation, apart from dosage effects, and the failure of ethylmethane sulphonate treatment to yield mutants, indicates that Ph1 has a complex structure. Here we have localized Ph1 to a 2.5-megabase interstitial region of wheat chromosome 5B containing a structure consisting of a segment of subtelomeric heterochromatin that inserted into a cluster of cdc2-related genes after polyploidization. The correlation of the presence of this structure with Ph1 activity in related species, and the involvement of heterochromatin with Ph1 (ref. 6) and cdc2 genes with meiosis, makes the structure a good candidate for the Ph1 locus.

Development of consistently crossable wheat genotypes for alien wheat gene transfer through fine-mapping of the Kr1 locus.

Breeders can force sexual hybridisation between wheat and related grass species to produce interspecific hybrids containing a dihaploid set of wheat and related chromosomes. This facilitates the introgression of desirable genes into wheat from the secondary gene pool. However, most elite European wheat varieties carry genes that suppress crossability, making the transfer of novel traits from exotic germplasm into elite wheat varieties difficult or impossible. Previous studies have identified at least five crossability loci in wheat. Here, the crossability locus with the largest effect, Kr1 on chromosome arm 5BL, was fine-mapped by developing a series of recombinant substitution lines in which the genome of the normally non-crossable wheat variety 'Hobbit sib' carries a recombinant 5BL chromosome arm containing segments from the crossable variety 'Chinese Spring'. These recombinant lines were scored for their ability to cross with rye over four seasons. Analysis revealed at least two regions on 5BL affecting crossability, including the Kr1 locus. However, the ability to set seed is highly dependent on prevailing environmental conditions. Typically, even crossable wheat lines exhibit little or no seed set when crossed with rye in winter, but show up to 90% seed set from similar crosses made in summer. By recombining different combinations of the two regions affecting crossability, wheat lines that consistently exhibit up to 50% seed set, whether crossed in the UK winter or summer conditions, were generated, thus creating a very important tool for increasing the efficiency of alien wheat transfer programmes.

Construction and analysis of a BAC library in the grass Brachypodium sylvaticum: its use as a tool to bridge the gap between rice and wheat in elucidating gene content.

A BAC library of 30,228 clones with an average insert size of 102 kb was constructed in the grass Brachypodium sylvaticum. Brachypodium has a simple genome, similar in size and repetitive DNA content to that of rice, and is more closely related than rice both to the major temperate cereals wheat and barley, and to the forage grasses. The library represents 6.6 genome equivalents, implying a 99.9% probability of recovering any specific sequence. The library was arrayed onto two high-density colony filters, which were screened with heterologous DNA probes from rice chromosome nine and from syntenous regions of wheat, barley, maize and oat. The construction of Brachypodium BAC contigs revealed that synteny between rice, wheat and Brachypodium was largely maintained over several regions of rice chromosome nine. This suggests that Brachypodium will be a useful tool in the elucidation of gene content in agronomically important cereal crops, complementing rice as a "grass genome model".